This invention relates generally to magnetic tape recording systems. More particularly, the invention relates to improved means and methods for more accurately controlling the speed of a magnetic tape in a reel-to-reel magnetic tape driving system.
One known approach for controlling tape velocity using a reel-to-reel drive mechanism requires the use of a prerecorded clock track on the tape. Clock pulses read from the clock track indicate the linear velocity of tape. If the tape velocity is too high, as indicated by more pulses being read in a preset time interval, a servo control system is used to slow down the angular velocity of the motor driving the take-up reel. This approach has the obvious disadvantage of requiring the use of a clock track on the tape.
Another known approach for maintaining constant tape velocity in a reel-to-reel tape drive system involves the use of a tachometer which is driven along with the tape. The tachometer signal indicates when the desired linear tape velocity has been reached and controls the rotational velocity of the take-up reel. Although this method eliminates the need for a clock track, slippage may occur between the tape and the tachometer apparatus, thereby reducing the accuracy of tape velocity control.
Another and particularly advantageous approach for providing tape velocity control with a reel-to-reel drive is disclosed in the commonly assigned U.S. Pat. No. 3,764,087. In a preferred embodiment described in this patent, optical sensing means are employed for producing a series of signals, the accumulated number of which is indicative of the number of revolutions of one of the reels. A binary counter which is set to zero at the beginning of winding of the tape counts these signals to produce a corresponding digital output signal representative of the number of turns of tape wound on the take-up reel. The counter counts up when the tape is moving in the forward direction and counts down when the tape is moving in the reverse direction. A digital-to-analog converter converts the digital output signal of the counter to an analog signal which is then applied to a linearity correcting circuit. The linearity correcting circuit is specially designed to modify the analog signal in a manner so as to cause the motor servo control unit to control the driving motor to rotate the reel at a velocity inversely proportional to increasing tape diameter, thereby maintaining a substantially constant linear velocity of tape.
Using the above described approach disclosed in U.S. Pat. No. 3,764,087, it has been found that the linearity correcting circuit can be adjusted with a precision alignment tape to provide a linear tape speed error of less than 1% over the entire length of the tape. However, as recognized in the aforementioned U.S. Pat. No. 3,764,087, the accuracy of this adjustment of the linearity correcting circuit is subject to variations in tape thickness as well as variations in the hub dimensions of the take-up reel. If a tape is used which has a varying thickness or a thickness different from that of the alignment tape, or if the hub dimensions are different, the linearity correcting circuit which was adjusted for a precision alignment tape and a given hub will no longer be able to provide the same accuracy of speed control, since such variations can cause the diameter of the tape wound on the take-up reel to be different for a given count than that obtained for the alignment tape. For example, a 10% difference in tape thickness from that of the alignment tape can cause an increasing speed error as the tape is wound amounting to approximately 6% when the tape is fully wound on the take-up reel.
Because of these inaccuracies inherent in the counter approach, the aforementioned U.S. Pat. No. 3,764,087 additionally presents an alternate embodiment employing an optical system for directly sensing tape diameter on a reel so as to thereby provide a signal to the servo control which more accurately corresponds to the actual diameter of the tape on the reel. However, as recognized in the patent, although this alternate optical approach is insensitive to hub or tape thickness, it is inherently less stable and more complex than the counter approach.